Emission Line and Continuum Variability in Active Galactic Nuclei

Summary

Abstract

Large-scale multiwavelength spectroscopic monitoring campaigns are producing new information about the central regions of AGNs. Reverberation mapping experiments are now being undertaken, and these are providing useful constraints on the structure and kinematics of the broad-line region and leading to reinvestigation of the physics of the line-emitting gas. In this contribution, the fundamental assumptions of reverberation mapping, some of the principal results of recent monitoring campaigns, and questions that have arisen from recent work are briefly reviewed.

Introduction

The broad emission lines in the spectra of AGNs vary in response to changes in the luminosity of the central ionizing source with a time delay due to light traveltime effects within the broad-line region (BLR). It is in principle possible to make use of these light travel-time effects to map out the geometry and kinematics of the BLR through detailed comparison of the continuum and emission-line variability. This technique, known as “reverberation” or “echo” mapping, requires large amounts of high-quality data. Recent campaigns to measure continuum and emission-line variations in AGNs are for the first time providing data suitable for this purpose. These new data are leading to important new inferences about the nature of the BLR and the central source. In this contribution, I will discuss progress made in application of reverberation mapping techniques and mention some of the areas where further progress can be made in the near future. A more complete review of the subject will be provided elsewhere.